The present disclosure relates to a transmission cable having an interlayer connection with a bump.
In various network systems and the like, data and the like are transmitted via a transmission cable (network cable), and a flat transmission cable allowing easy handling of wiring has been developed. The flat transmission cable is made by performing a patterning processing to form a signal line using a method similar to that used for a so-called flexible wiring board, and is advantageous in reducing irregularities when the flat transmission cable is wired at an arbitrary location such as under floor furnishings and the like.
In the mean time, a broadband era has necessitated a fast and large-capacity transmission, and thus the improvements in transmission cables are also desired. For example, the transmission cable made by merely forming a signal line involves problems such as malfunctions caused by noise, interference of signals, and the like.
Under such circumstances, a transmission cable has been suggested in which a ground line is arranged between signal lines to be electrically connected to a shield layer formed via an insulation layer (for example, see, Japanese Patent Application Publication No. H11-162267, Japanese Patent Application Publication No. 2002-117726, and the like). Specifically, Japanese Patent Application Publication No. H11-162267 discloses a flat cable in which a shield pattern and a shield layer are electrically connected with an electrically-conductive bump formed by an electrically-conductive resin paste. Japanese Patent Application Publication No. 2002-117726 discloses a sheet flexible cable in which a sheet shield conductor and a sheet insulator are laminated, wherein a shield ground line among multiple conducting wires arranged parallel in the insulator is connected to the shield conductor via an electrically-conductive through-hole and the like. It should be noted that a similar structure can be seen in a circuit board structure (see, for example, Japanese Patent Application Publication No. 7-99397 and the like).
Prior art transmission cables, however, have disadvantages that an electric resistance of the conductive paste is high and that a connection resistance with a copper foil making up a ground line is also high. If these electric resistance and connection resistance are high in the transmission cable, the ground line will not function sufficiently, and accordingly, a noise and an interference between signal lines cannot be sufficiently suppressed. This becomes a major obstacle in a signal transmission speedup and the like.
A problem of increasing a manufacturing cost arises in a case where a manufacturing method is employed that involves forming an electrically-conductive bump using a conductive paste and pressing the electrically-conductive bump into an insulator layer to penetrate therethrough. For example, Japanese Patent Application Publication No. H11-162267 discloses a technology to form an electrically-conductive bump according to a stamp method and the like that involves pressing an electrically-conductive composition (paste), contained in a casing, onto a predetermined position of a stainless plate and pushing out the electrically-conductive composition from holes of a mask. Such a method requires a dedicated apparatus such as a stamp apparatus, and furthermore requires, e.g., to perform a heat hardening processing upon repeatedly performing a printing and drying processing at an identical position, thus resulting in complicated steps.